describe nice level option, closes #140

Author: toy

CHANGELOG.markdown

@@ -2,6 +2,8 @@
 
 ## unreleased
 
+* Describe `nice` level option [#140](https://github.com/toy/image_optim/issues/140) [@toy](https://github.com/toy)
+
 ## v0.24.1 (2016-11-20)
 
 * Use `image_size ~> 1.5` with `apng` detection, so apng images are not optimised to one frame version [#142](https://github.com/toy/image_optim/issues/142) [@toy](https://github.com/toy)

README.markdown

@@ -263,7 +263,7 @@ optipng:
 
 ## Options
 
-* `:nice` — Nice level *(defaults to `10`)*
+* `:nice` — Nice level, priority of all used tools with higher value meaning lower priority, in range `-20..19`, negative values can be set only if run by root user *(defaults to `10`)*
 * `:threads` — Number of threads or disable *(defaults to number of processors)*
 * `:verbose` — Verbose output *(defaults to `false`)*
 * `:pack` — Require image\_optim\_pack or disable it, by default image\_optim\_pack will be used if available, will turn on `:skip-missing-workers` unless explicitly disabled *(defaults to `nil`)*

lib/image_optim/runner/option_parser.rb

@@ -133,7 +133,9 @@ def wrap_regex(width)
     options[:threads] = threads
   end
 
-  op.on('--[no-]nice N', Integer, 'Nice level (defaults to 10)') do |nice|
+  op.on('--[no-]nice N', Integer, 'Nice level, priority of all used tools '\
+      'with higher value meaning lower priority, in range -20..19, negative '\
+      'values can be set only if run by root user (defaults to 10)') do |nice|
     options[:nice] = nice
   end